High velocity pneumatic devices are used in construction work to excavate or dislodge soil from around electrical cables, water pipes, gas mains and the like. The pneumatic device has the advantage of being capable of pulverizing the soil without damaging the utility lines.
The typical high velocity pneumatic device consists of a body or housing which carries an elongated tube. The body is connected to a source of air under pressure, such as a compressor, and a valve mechanism mounted within a passage in the body controls the flow of air to the tube.
Mounted in the distal end of the tube is a nozzle which is designed to increase the velocity of the air being discharged from the tube. More specifically, the typical nozzle is provided with an inwardly converging upstream end which merges into a diverging downstream end. This configuration acts to reduce the pressure of the air or gas and increases the velocity, so that in practice the air being discharged from the tube can have a velocity up to about 2200 feet per second.
European patent application 0 251 660 describes a typical high velocity pneumatic device having particular use for excavating soil. The device as described in the aforementioned patent application includes a complicated valve mechanism, including both a pilot valve and a main control valve. Manual operation of the trigger by the operator will open the pilot valve which generates a signal that is transmitted to the main valve to open the main valve and supply compressed air through the discharge tube. It has been found that dirt and foreign material can readily clog the openings in the pilot valve, requiring frequent disassembly and cleaning of the valve. When disassembled for cleaning, the pilot valve is difficult to realign on assembly, thus requiring substantial down-time for the cleaning operation.
Pneumatic devices as used in the past have also included a discharge tube composed of two or more connected sections. The connections between the sections results in an inwardly protruding joint which produces turbulence in the air flow, thus reducing the discharge velocity.
As a further problem, the high velocity pneumatic devices as used in the past have generally included a steel discharge tube with a brass nozzle being mounted in the distal end of the tube. While the brass nozzle is non-sparking, the steel tube is electrically conductive which can pose a hazard to the operator in the event the device is employed to excavate soil around a ruptured electrical cable.
Therefore, there has been a need for a simple and inexpensive high velocity pneumatic device which eliminates the possibility of explosions or the transmission of electrical current during use.